[ownedbycats]

First off, check out the template documentation. Bookmark it too.

I'll try to explain a few things with one of my own templates.

Here's part of a somewhat simple GPM template made to display icons for various fanfics rather than a zillion different column icon rules:

program:
	f = $#fanficcat;

   strcat(
		if "^Fallout$" inlist f then 'fallout.png:' fi,
		if "^Half-Life$" inlist f then 'halflife.png:' fi,
		if "^(Mass Effect Trilogy|Mass Effect: Andromeda)$" inlist f then 'masseffect.png:' fi,
		if "^The Elder Scrolls$" inlist f then 'theelderscrolls.png:' fi,
		if "^Pokémon$" inlist f then 'pokemon.png:' fi,		
		if "^Portal$" inlist f then 'portal.png:' fi,
   )

Defining things:

f = $#fanficcat;

Here, I am telling the template that 'f' means the value of the #fanficcat column. A column 'field reference' can be preceded with $ or $$; the former is input, while the latter is 'raw' input (the latter mostly matters for datetimes, which have a lot of different formattings).

While in the fanfic icon example I'm just using it as shorthand, here's another example from a more complicated template.

Here, I use regular expressions to extract two numbers (defined as 'f' and 's') from a column, do arithmetic on them, and then define the results of the math as 'newpercent.' This lets me use it further down the template.

f = re($#chapters, '(.*)/.*', '\1');
s = re($#chapters, '.*/(.*)', '\1');
newpercent = round(multiply ((f / s), 100));

GPM if-then templates:

if "^Fallout$" inlist f then 'fallout.png:' fi

This is an if-then template. If 'Fallout' is found in the list f, then output 'fallout.png:' and finish.

You can also throw elses in there:

if "^Fallout$" inlist f then 'fallout.png:' else 'null.png' fi

Here, if 'Fallout' is found, it'll output 'fallout.png'; otherwise, it'll output 'null.png:' and then finish.

Concatenation:

You notice that all the if-then templates are wrapped in a strcat() and that there's a comma at the end of each one.

This means that anything inside it will all be concatenated together. So if I have a book with "Fallout" and "Half-Life" in the column, instead of only displaying the first one, it'll output "fallout.png:halflife.png:" (and display both of them).